Spinal cord stimulation (SCS) is a procedure to treat chronic pain wherein a target tissue is stimulated by an electrical lead. SCS is commonly used to treat failed back surgery syndrome and peripheral ischemic pain after more conservative therapies have failed. The lumbar spinal cord is one example of a target tissue which may be subjected to a low intensity electric current from a stimulation lead to treat chronic pain. The amplitude of electric current is on the order of milliamps or volts, and the frequency of the electric current is commonly between 20 and 120 hertz.
While the exact effect of the SCS procedure is not completely understood, it is theorized that the electric stimulation of tissues such as the lumbar spinal cord suppress the excitability of neurons and some amino acids. An electrical stimulator device in its most basic form is a stimulator lead, electrodes disposed on a distal end of the lead, and a source of electrical power interconnected to the stimulation lead. The source of electrical power may be an implantable pulse generator (IPG) or a radio frequency (RF) receiver that receives power from an external transmitter.
A similar type of treatment for chronic pain utilizes radio-frequency energy to induce a thermal lesion in the target tissue. In this type of procedure, the therapeutic benefit is intended to derive from heating the target tissue and not from immersing the tissue in an electrical field. Thus, the electrical lead in this treatment is strictly for use in heating the tissue, and there is no therapeutic electrical field generated.
For both electrical and thermal stimulation, an electrical current generator, commonly referred to as a pulse generator, may be used to transmit a pulse of electrical current to an implanted stimulation lead that has been accurately placed to transmit the electrical or thermal energy from the electrodes to the target tissue in order to treat the particular condition. Implanted pumps and generators can be used to deliver the electrical stimulation as opposed to transdermal delivery devices. More particularly, IPGs are commonly used so that patients do not have to return to a medical facility each time treatment is to be conducted.
The intervertebral disc (IVD) provides separation, shock absorption, and controlled motion between vertebral bodies. The disc is comprised of a central nucleus of a semi-fluid mass of mucoid material, (nucleus pulposus), an outer more dense collagen ring (annulus fibrosis), and a thin, metabolically active cellular layer separating the nucleus and the outer collagen ring, referred to as the annular nuclear interface/transitional zone. Disc nutrition is tenuous at best and is provided by diffusion through the vertebral end plate in contact with the outer surface of the disc. As a result, a disc has limited ability to heal or regenerate. Due to age, injury or other conditions, cracks or fissures may develop in the wall of invertebral discs causing a chronic source of pain in many patients. Additionally, the inner disc tissue (nucleus) will frequently cause the disc to bulge or herniate into the fissures in the outer region of the disc, thus causing nerve tissue therein to generate pain signals.
Placement of a stimulation lead within a disc can be quite difficult. Because a disc does not have a uniform density, stimulation leads can be quite difficult to place and may require the attending physician to make multiple attempts for proper placement or abandon the procedure. Of course, multiple placement attempts greatly increase the invasive nature of the procedure and therefore create unnecessary tissue damage and increased risk of other ill effects. Inability to perform the procedure denies the patient a therapeutic option. Improper placement of the stimulation lead can also result in the undesirable damage of nerve tissue that is not contributing to the chronic pain or other ailments.
Medical practitioners use magnetic resonance imaging (MRI) machines to help accurately locate the stimulation lead to avoid too many placement attempts. MRI machines commonly use two magnetic fields to orient dipolar molecules for imaging: a pulsed magnetic-gradient field and a pulsed radio-frequency field.
The components of a stimulator can form a loop that induces an electrical current from the MRI's magnetic fields. Excess length of the stimulation lead or conducting wire is often looped to organize the wire such that it does not interfere with other devices or processes. Looped conducting wire also provides a variable length between the distal end of the stimulation lead and the pulsed generator so that a patient can move after a surgery without relocating the lead or generator.
The induction of an electrical current during an MRI procedure can result in severe burns to the patient, who is often under an anesthetic and cannot sense a burning sensation of the electrical current. Examples of prior art devices that address electrical induction in a lead may be found in U.S. Pat. Nos. 8,676,340 and 8,688,226. However, these references do not address electrical induction in a looped stimulation lead or conducting wire. Therefore, there is a need for an electrical stimulation device that can be used in an MRI machine without inducing an electrical current and burning a patient.